4.4 Vadose zone hydrology
3 min read•july 22, 2024
The , spanning from the land surface to the water table, plays a crucial role in the hydrologic cycle. It regulates water , stores moisture for plants, and facilitates groundwater recharge. Understanding this zone is key to managing water resources effectively.
Unsaturated flow in the vadose zone is driven by and gravity. The Richards equation models this complex process, combining Darcy's law with mass conservation. This knowledge helps predict water movement and , impacting groundwater management and environmental protection.
The Vadose Zone and Unsaturated Flow
Vadose zone in hydrologic cycle
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- Vadose zone (unsaturated zone) extends from land surface to water table
- Contains both air and water in pore spaces (soil, rock)
- Regulates infiltration of water from land surface into subsurface
- Controls amount and timing of water entering soil
- Stores water in soil matrix
- Available for plant uptake (transpiration) or evaporation back to atmosphere
- Allows water to percolate downward and recharge groundwater aquifers
- Replenishes water in below water table
Processes of unsaturated flow
- Unsaturated flow driven by capillary forces and gravity
- Capillary forces attract water molecules to soil particles causing upward movement against gravity ()
- Gravity pulls water downward through soil profile ()
- Capillary rise moves water upward from water table into unsaturated zone
- Height of rise depends on soil pore size distribution
- Finer-grained soils (clay) have smaller pores and higher capillary rise
- Coarser-grained soils (sand) have larger pores and lower capillary rise
- Height of rise depends on soil pore size distribution
- Preferential flow paths are channels or macropores allowing rapid water movement through vadose zone
- Caused by root channels, animal burrows, or soil cracks
- Bypass soil matrix leading to fast transport of water and contaminants to water table
Modeling Unsaturated Flow and Its Implications
Richards equation for soil water
- Richards equation describes water movement in unsaturated soils
- Combines Darcy's law for fluid flow with conservation of mass
- Equation:
- = volumetric water content
- = time
- = vertical coordinate (positive upward)
- = (depends on water content)
- = matric potential (represents capillary forces)
- Solving requires soil hydraulic properties
- Water retention curve and unsaturated function
- Measured in lab or estimated from soil texture and structure
Vadose zone's environmental impact
- Controls rate and timing of groundwater recharge
- Recharge occurs when water percolates through vadose zone to water table
- Amount and timing depend on precipitation, evapotranspiration, soil properties
- Acts as buffer for contaminants slowing transport to water table
- Contaminants can be adsorbed to soil particles, degraded by microbes, or diluted by dispersion
- But preferential flow paths can bypass soil matrix allowing faster contaminant transport
- Understanding vadose zone crucial for:
- Sustainably managing groundwater resources
- Assessing aquifer vulnerability to contamination
- Designing remediation strategies for contaminated sites
Key Terms to Review (20)
Agricultural runoff: Agricultural runoff refers to the water that flows over agricultural land and carries away dissolved nutrients, pesticides, and other contaminants into nearby water bodies. This process often occurs after rainfall or irrigation, as excess water collects and moves across fields, picking up pollutants along the way. The impacts of agricultural runoff are significant, affecting both vadose zone hydrology and contributing to various types of water pollution.
Aquifer Recharge: Aquifer recharge is the process by which water from precipitation, surface water, or artificial sources infiltrates the ground and replenishes an aquifer. This crucial process is essential for maintaining groundwater supplies and is influenced by various factors such as soil type, land use, and climate. Understanding aquifer recharge helps to manage water resources sustainably and highlights the interconnectedness of groundwater and surface water systems.
Baseflow: Baseflow is the portion of streamflow that is sustained by groundwater discharge, providing a continuous flow of water in rivers and streams during dry periods. It plays a critical role in maintaining river ecosystems and water supply, especially during low flow conditions when precipitation is minimal.
Capillary Forces: Capillary forces are the forces of attraction between liquid molecules and solid surfaces, which play a crucial role in the movement of water through porous materials like soil. These forces arise due to the surface tension of water and the adhesive forces between water and soil particles, enabling water to move upward or laterally against the force of gravity, particularly in unsaturated soils. Understanding capillary forces is essential for grasping how water interacts with soil, influencing both infiltration rates and vadose zone hydrology.
Capillary rise: Capillary rise is the phenomenon where water moves upward against gravity through small pores or spaces in soil or other materials due to surface tension and adhesive forces between water molecules and the surrounding material. This process is crucial for the movement of water in the vadose zone, affecting moisture availability for plants and influencing groundwater recharge.
Contaminant transport: Contaminant transport refers to the movement of harmful substances through various media, such as water, soil, and air, due to processes like advection, dispersion, and diffusion. Understanding how contaminants move is crucial for assessing their impact on the environment, especially in the vadose zone, where water moves through unsaturated soil and interacts with pollutants before reaching groundwater. This concept is key to evaluating risks to water supplies and ecosystems.
Field Capacity: Field capacity refers to the maximum amount of water that soil can hold after excess water has drained away and the rate of downward movement has decreased. It plays a crucial role in understanding how water is retained in the vadose zone, influences soil moisture dynamics, and affects overland and channel flow processes. This concept is essential in determining the availability of water for plant use and helps predict how water moves through the landscape during precipitation events.
Green-Ampt Model: The Green-Ampt model is an infiltration equation used to estimate the rate at which water enters the soil surface. This model helps to quantify how much water can infiltrate before saturation occurs and is crucial for understanding soil-water dynamics, especially in the vadose zone where unsaturated flow is present. It also plays an important role in evaluating how different variables influence infiltration processes and runoff generation during rainfall events.
Hydraulic Conductivity: Hydraulic conductivity is a measure of a material's ability to transmit water through its pores or fractures, crucial for understanding groundwater flow and its interaction with surface water. It relates to aquifer properties, influencing how quickly water can move through soil and rock, which is essential for managing groundwater resources and recharge processes.
Infiltration: Infiltration is the process by which water on the ground surface enters the soil, allowing it to move downward through the soil layers. This process is crucial in determining soil moisture levels, groundwater recharge, and the overall movement of water in the hydrologic cycle.
Nitrate leaching: Nitrate leaching is the process by which nitrate ions ($$NO_3^-$$) are washed out of the soil and into groundwater or surface water, often as a result of precipitation or irrigation. This process can lead to nutrient depletion in the soil and can contaminate drinking water sources, making it an important concern in agricultural and environmental contexts. Understanding this phenomenon is crucial for managing soil health, water quality, and overall ecosystem sustainability.
Percolation: Percolation is the process by which water moves through soil and porous rock, primarily due to gravity, allowing it to filter down from the surface into deeper layers. This movement plays a critical role in various hydrologic processes, influencing how water is stored and transmitted within the soil and affects groundwater recharge, soil moisture dynamics, and overall water availability.
Richards' Equation: Richards' Equation is a partial differential equation that describes the movement of water in unsaturated soils, taking into account the effects of soil moisture retention and water flow dynamics. This equation is essential in vadose zone hydrology as it models how water infiltrates through the soil and how it interacts with soil properties and atmospheric conditions. Understanding this equation helps in predicting groundwater recharge and managing water resources effectively.
Saturated Zone: The saturated zone is the region of soil or rock beneath the earth's surface where all the pores and fractures are completely filled with water. This area plays a crucial role in groundwater systems, as it stores water that can be tapped for human use and maintains the hydrological balance of ecosystems above.
Soil moisture sensors: Soil moisture sensors are devices used to measure the water content in soil, providing crucial data for understanding the hydrological processes occurring in the vadose zone and for managing drought conditions. These sensors can help determine when irrigation is needed, optimize water use, and enhance agricultural practices by monitoring soil water availability. They play a significant role in both assessing soil health and implementing effective drought management strategies.
Stormwater management: Stormwater management refers to the strategies and practices employed to control the quantity and quality of stormwater runoff, preventing flooding, erosion, and pollution. Effective stormwater management aims to mimic natural hydrological processes, allowing rainwater to infiltrate into the ground and reduce surface runoff, which is essential in maintaining water quality and protecting aquatic ecosystems.
Tensiometers: Tensiometers are instruments used to measure the tension or matric potential of soil water, providing crucial data on the moisture status within the vadose zone. They help in understanding how water moves through soil and its availability to plants, making them essential tools for hydrologists and agricultural scientists studying unsaturated zones.
Unsaturated Hydraulic Conductivity: Unsaturated hydraulic conductivity is a measure of how easily water can move through soil or porous media when the soil is not fully saturated with water. This term is crucial for understanding the movement of water in the vadose zone, which lies above the groundwater table and plays a vital role in water infiltration, soil moisture dynamics, and plant water availability. The rate at which water can move through unsaturated soils affects various processes, such as evaporation, transpiration, and drainage, making it an essential concept in hydrology.
Vadose zone: The vadose zone, also known as the unsaturated zone, is the area of soil and rock above the groundwater table where the pores are not fully saturated with water. This zone plays a crucial role in hydrology as it acts as a buffer between the land surface and the groundwater below, influencing water movement, soil moisture availability, and nutrient transport.
Wilting Point: Wilting point refers to the soil moisture content at which plants can no longer extract water from the soil, leading to plant wilting and potential death if water is not replenished. It is a critical concept in understanding how plants interact with their environment, especially concerning soil moisture availability and plant health, highlighting the delicate balance of water retention in soil profiles.